The present invention relates to a liquid crystal display. More particularly, it relates to a TFT liquid crystal display that allows display of high picture quality to be embodied with the use of a low voltage driving circuit. Also, it relates to the driving circuit therefor.
Referring to FIG. 2 and FIGS. 3A, 3B, the explanation will be given below concerning a conventional TFT liquid crystal display.
FIG. 2 is a block diagram for illustrating the conventional TFT liquid crystal display. Also, FIGS. 3A, 3B illustrate driving waveform diagrams for the conventional liquid crystal display.
In FIG. 2, a reference numeral 201 denotes an interface signal including display data and a synchronization signal that are transferred from a system (not illustrated). A numeral 202 denotes an interface circuit for generating display data and control signals that drive the conventional liquid crystal display. A numeral 203 denotes a signal driving circuit for generating a tone voltage corresponding to the display data. A numeral 204 denotes a scan driving circuit for selecting scanning lines in sequence. A numeral 205 denotes a power supply circuit for generating a power supply necessary for the operation of each block. Also, a numeral 206 denotes a liquid crystal panel on which there is executed a display corresponding to the display data inputted.
Of the signals that the interface circuit 202 generates, a numeral 207 denotes a control signal that controls the signal driving circuit 203 and includes the display data and the synchronization signal. A numeral 208 denotes a control signal that controls the scan driving circuit 204 and transfers a timing signal for scanning the scanning lines in sequence. A numeral 209 denotes a liquid crystal-applying voltage alternating signal xe2x80x9cMxe2x80x9d that is transferred to the power supply circuit 205.
Of the signals that the power supply circuit 205 generates, a numeral 210 denotes a tone voltage signal transferred to the signal driving circuit 203. The tone voltage signal 210 transfers a voltage that functions as a reference voltage of the tone voltage corresponding to the display data transferred to the liquid crystal panel 206. A numeral 211 denotes a scanning voltage signal transferred to the scan driving circuit 204. A numeral 212 denotes an opposed electrode voltage feeding line that transfers an opposed electrode voltage xe2x80x9cVcomxe2x80x9d to an opposed electrode of a liquid crystal 217 and an opposed electrode of a compensation capacitor 218. Here, the liquid crystals 217 and the compensation capacitors 218 constitute the liquid crystal panel 206. Moreover, a numeral 213 denotes a group of signal lines for transferring the tone voltage corresponding to the display data generated in the signal driving circuit 203. A numeral 214 denotes a group of scanning lines for transferring a scanning voltage that switches, into a selection or a non-selection state, each of the scanning lines generated in the scan driving circuit 204. A numeral 215 denotes pixels constituting the liquid crystal panel 206. The pixels 215 are formed at the intersection points of the group of signal lines 213 and the group of scanning lines 214, and thus the liquid crystal panel 206, eventually, has a matrix structure. Also, the numbers of the pixels 215 in the horizontal and the vertical directions are equivalent to the horizontal and the vertical resolutions. Incidentally, in general, in the case of a color liquid crystal display, the three primary colors, i.e., red, green and blue colors, constitute one pixel thereof. Accordingly, when the respective color pixels are arranged in the horizontal direction, the number of the pixels in the horizontal direction becomes equal to 3 times as large as the horizontal resolution. Also, the configuration employed commonly is that the pixels 215 arranged in the horizontal direction share one scanning line of the group of scanning lines 214 and the pixels 215 arranged in the vertical direction share one signal line of the group of signal lines 213. Furthermore, reference numerals within each of the pixels 215 denote the following components: 216 a thin film transistor (hereinafter, referred to as xe2x80x9cTFTxe2x80x9d), i.e., a switching element, 217 the liquid crystal, 218 the compensation capacitor, 219 a source electrode, 220 a between-gate/source parasitic capacitor configured between the scanning line (this is also referred to as xe2x80x9cgate linexe2x80x9d) 214 and the source electrode 219.
In FIGS. 3A, 3B, reference notations Vg(n), Vg(n+1) denote, of the group of scanning lines 214 illustrated in FIG. 2, driving waveforms of scanning lines that drive the nth line and the (n+1)th line, respectively. In addition, a notation Vgon denotes a selection voltage level, and a notation Vgoff denotes a non-selection voltage level. The notation Vcom indicates an ideal driving waveform of the opposed electrode voltage feeding line 212, and a notation Vcomh denotes a high electric potential voltage level and a notation VcomL denotes a low electric potential voltage level. A notation Vd indicates the tone voltage of the group of signal lines 213. When, with reference to the opposed electrode voltage Vcom, the tone voltage Vd is positioned on the negative polarity side, a voltage with negative polarity is applied to the pixel 215. Conversely, when the tone voltage Vd is positioned on the positive polarity side, a voltage with positive polarity is applied to the pixel 215. In the liquid crystal display, the electric potential difference between the opposed electrode voltage Vcom and the tone voltage Vd becomes equal to an effective voltage value applied to the liquid crystal 217. The effective voltage value causes the liquid crystal display to operate in such a manner that the luminance thereof is varied. Moreover, in the present embodiment, the explanation will be given based on the following characteristic: If the electric potential difference between the opposed electrode voltage Vcom and the tone voltage Vd is small, the display becomes a dark display (the example: black display) and if the electric potential difference is large, the display becomes a light display (the example: white display). In FIGS. 3A, 3B, the drain voltage Vd is the tone voltage executing the white display, and a notation VdWH denotes a white display drain voltage with positive polarity, and a notation VdWL denotes a white display drain voltage with negative polarity. When, with reference to the opposed electrode voltage Vcom, the drain voltage is positioned on the negative polarity side, an effective voltage value Vrms 1 is applied to the pixel. Conversely, when the drain voltage is positioned on the positive polarity side, an effective voltage value Vrms 2 is applied to the pixel.
Referring to FIG. 2 once again, the detailed explanation will be given below concerning the operation of the conventional TFT liquid crystal display.
The interface circuit 202 inputs the display data and the synchronization signal transferred by the interface signal 201. Then, the interface circuit 202 generates and outputs the control signal 207 to the signal driving circuit 203, the control signal 208 to the scan driving circuit 204, and the liquid crystal-applying voltage alternating signal M 209 to the power supply circuit 205. The signal driving circuit 203 fetches in sequence the display data by the amount of one horizontal line, using the display data and the synchronization signal transferred by the control signal 207. Then, after fetching the display data by the amount of one horizontal line, the signal driving circuit 203 outputs, simultaneously from the group of signal lines 213, the tone voltage corresponding to the fetched display data by the amount of one horizontal line. The signal driving circuit 203 continues outputting the tone voltage by the amount of one horizontal line during one horizontal time-period. Also, at this time, in parallel to the continuous outputting of the tone voltage, the signal driving circuit 203 executes an operation of fetching in sequence the display data of the next horizontal line.
Accordingly, it turns out that the display data that the interface circuit 202 outputs is converted into the tone voltage, then being outputted to the liquid crystal panel 206 during the next horizontal time-period. The signal driving circuit 203 repeats this operation, thereby outputting, to the liquid crystal panel 206, the tone voltage corresponding to the display data by the amount of one frame, i.e., by the amount of one screen. Also, the tone voltage that the signal driving circuit 203 outputs is generated by employing, as a reference voltage, the tone voltage transferred by the tone voltage line 210. In general, the reference voltage of the tone voltage transferred by the tone voltage line 210 is a voltage including a plurality of levels that range from the voltage for the black display to the voltage for the white display. Furthermore, the scan driving circuit 204 synchronizes with the control signal 208, thus applying a selection voltage to the group of scanning lines 214 from the 1st line in sequence. At this time, the selection voltage is applied to the TFT 216 in each of the pixels 215. As the result, the TFT 216 is switched into the selection state, thereby applying the tone voltage, which is transferred from each of the group of signal lines 213, to the liquid crystal 217 and the compensation capacitor 218. Then, if a non-selection voltage is applied to each of the scanning lines 214, the resultant non-selection state is maintained until it is switched back into the selection state next. In this way, in the liquid crystal display, the scanning is controlled in the sequence of the lines, and an amount of the light passing through the liquid crystal 217 is controlled at a voltage level of the effective voltage value applied to the liquid crystal 217. These controls have made it possible to embody the tone display.
Referring to FIGS. 3A, 3B once again, the explanation will be given below in more detail concerning the operation of applying the electric voltage to the liquid crystal 217 in the pixel 215. As illustrated in FIGS. 3A, 3B, if the selection voltage Vgon is applied to the scanning line G(n), the TFT 216 illustrated in FIG. 2 is switched into the xe2x80x9cONxe2x80x9d state. Then, as explained earlier, the drain (tone) voltage Vd transferred by the signal line 213 is applied to the liquid crystal 217 in the pixel 215. Meanwhile, if the non-selection voltage Vgoff is applied to the scanning line G(n), the TFT 216 is switched into the xe2x80x9cOFFxe2x80x9d state with this timing, and the voltage thereof is maintained. In the timing with which the selection voltage Vgon is applied to the scanning line G(n), the voltage level of the opposed electrode voltage feeding line 212 is the low electric potential voltage VcomL (negative polarity). Accordingly, the voltage applied to the liquid crystal turns out to be the voltage with positive polarity (The white display drain voltage is VdWH.). Similarly, in the timing with which the selection voltage Vgon is applied to the scanning line G(n+1), the voltage level of the opposed electrode voltage feeding line 212 is the high electric potential voltage VcomH (positive polarity). Accordingly, the voltage applied to the liquid crystal turns out to be the voltage with negative polarity (The white display drain voltage is VdWL.).
Generally speaking, it is required to apply an alternating voltage to the liquid crystal with a period of one frame (about 60 Hz). Consequently, in a line corresponding to each of the group of the scanning lines 214, it is required to apply, in a timing with which the voltage is applied next, a voltage the polarity of which is opposite to that of the voltage applied before. Furthermore, if the polarity of the tone voltage applied all over the surface of one screen is biased onto either of the two types of polarities, there occurs a flickering phenomenon called xe2x80x9cflickerxe2x80x9d. Accordingly, in the present embodiment, an alternating driving for each line has been embodied so that the tone voltage with positive polarity and that with negative polarity will be able to be applied for each line. Consequently, the voltage level of the opposed electrode voltage feeding line 212 alternates for each line the high electric potential voltage VcomH (positive polarity) and the low electric potential voltage VcomL (negative polarity).
The characteristic of the present conventional driving system is as follows: The present conventional driving system can be configured using the signal driving circuit 203 that has a dynamic range of the tone voltage Vd illustrated in FIGS. 3A, 3B, i.e., a withstand voltage that the tone voltage with either of the polarities can generate. This is made possible by the fact that, in the driving system, the opposed electrode voltage Vcom of the opposed electrode voltage feeding line 212 has been alternated. In contrast to this, in general, when applying the tone voltage with positive polarity and that with negative polarity to the liquid crystal, a signal driving circuit becomes necessary that has a dynamic range 2 times as long as the above-described dynamic range illustrated in FIGS. 3A, 3B.
Next, referring to FIGS. 4A, 4B to FIGS. 7A, 7B, the explanation will be given below regarding problems in the conventional liquid crystal display. FIGS. 4A, 4B illustrate driving waveform diagrams in the respective portions in the case where the white display is performed by the conventional liquid crystal display. FIGS. 5A, 5B illustrate driving waveform diagrams in the respective portions in the case where the black display is performed by the conventional liquid crystal display. FIG. 6 illustrates an example of a displayed screen at the time of being displayed by the conventional liquid crystal display. FIGS. 7A, 7B illustrate driving waveform diagrams in the case where the example of the displayed screen illustrated in FIG. 6 is displayed by the conventional liquid crystal display.
Both of FIGS. 4A, 4B illustrate operations of applying the white display voltage. FIG. 4A illustrates an example where the tone voltage with negative polarity is applied. FIG. 4B illustrates an example where the tone voltage with positive polarity is applied. In FIG. 4A, a reference notation Vg denotes a voltage waveform applied to the respective scanning lines. In addition, Vgon denotes the selection voltage level, and Vgoff denotes the non-selection voltage level. Vd indicates the tone voltage waveform applied to the respective signal lines. VdWH denotes the white display voltage with positive polarity, and VdWL denotes the white display voltage with negative polarity. Vcom 1 denotes an opposed electrode voltage waveform inputted into the liquid crystal panel 206, and Vcom 2 denotes the opposed electrode voltage waveform inside the liquid crystal panel 206. Vs denotes a source voltage waveform of the source electrode 219 in the pixel 215 inside the liquid crystal panel 206. Also, the reference notations in FIG. 4B are the same as those in FIG. 4A.
Both of FIGS. 5A, 5B illustrate operations of applying the black display voltage. FIG. 5A illustrates an example where the tone voltage with negative polarity is applied. FIG. 5B illustrates an example where the tone voltage with positive polarity is applied. In FIG. 5A, Vd indicates the tone voltage waveform applied to the respective signal lines. In addition, VdBH denotes a black display voltage with positive polarity, and VdBL denotes a black display voltage with negative polarity. The other waveforms are the same as those of the driving voltages illustrated in FIG. 4A. Also, the reference notations in FIG. 5B are the same as those in FIG. 5A.
FIG. 6 presents the example of the displayed screen in the case where an intermediate luminance is displayed all over the entire screen and a white rectangle is displayed at the central portion. In FIG. 6, there is illustrated a phenomenon that the luminances come to differ between intermediate luminance display regions xe2x80x9cBxe2x80x9d where the white rectangle is not displayed and display regions xe2x80x9cAxe2x80x9d that are located on the right and the left sides of the white rectangle. This phenomenon is a picture quality deterioration called xe2x80x9ctransverse smearxe2x80x9d, which, in the conventional liquid crystal display, occurs in the case of the low voltage driving that alternates the opposed electrode voltage to be applied to the opposed electrode voltage feeding line.
FIGS. 7A, 7B illustrate driving waveform diagrams in the respective portions in the example of the displayed screen illustrated in FIG. 6. FIG. 7A illustrates the driving waveform diagram in the intermediate tone display regions xe2x80x9cBxe2x80x9d illustrated in FIG. 6, and FIG. 7B illustrates the driving waveform diagram in the intermediate tone display regions xe2x80x9cBxe2x80x9d illustrated in FIG. 6. Additionally, in the present conventional embodiment, there are described both of the examples where the tone voltage with negative polarity is applied. In FIG. 7A, Vd indicates a voltage waveform in the intermediate luminance display. VdGH denotes an intermediate luminance display voltage with positive polarity, and VdGL denotes an intermediate luminance display voltage with negative polarity. The other waveforms are the same as those of the driving voltages illustrated in FIG. 4A. Also, the reference notations in FIG. 7B are the same as those in FIG. 7A.
Next, the detailed explanation will be given below regarding the occurrence mechanism of the transverse smear, i.e., the phenomenon that the variation in the luminance occurs in the display regions xe2x80x9cAxe2x80x9d that are located on the right and the left sides of the white rectangle as illustrated in FIG. 6.
In the case of the low voltage driving that alternates the opposed electrode voltage to be applied to the opposed electrode voltage feeding line, since the opposed electrode voltage feeding line is common to all the pixels, there can be said the following: When the opposed electrode voltage is the high electric potential voltage (positive polarity), in all the pixels on the lines into which the tone voltage is to be written, the tone voltage with negative polarity is applied. Also, when the opposed electrode voltage is the low electric potential voltage (negative polarity), in all the pixels on the lines into which the tone voltage is to be written, the tone voltage with positive polarity is applied. As the result, it turns out that there occurs an electric current concentration. This electric current concentration is such that, through the liquid crystals 217 and the compensation capacitors 218 in all the pixels 215, electric currents in all the pixels flow into or out of the opposed electrode voltage feeding line in one direction. At this time, a time constant of the opposed electrode voltage feeding line exerts influences, thereby causing a distortion to occur in the opposed electrode voltage. FIGS. 4, 5 reflect and illustrate the manner of this process.
In the driving waveform diagrams in FIGS. 4A, 4B and FIGS. 5A, 5B, as illustrated in FIG. 2, there exists the between-gate/source parasitic capacitor 220 in each of the pixels 215 in the liquid crystal panel 205. Capacitance of the parasitic capacitor 220 is caused by the TFT 216, and thus is concerned with insulating films and silicon (the both are not illustrated) of the TFT 216. When the scanning line is transitioned from the selection state to the non-selection state, the TFT 216 is switched from the xe2x80x9cONxe2x80x9d state to the xe2x80x9cOFFxe2x80x9d state. At this time, the voltage that has been applied to the liquid crystal 217 and the compensation capacitor 218 is distributed to the parasitic capacitor 220 as well, since the above-described insulating films and silicon can also be regarded as capacitors.
Assuming that the capacitance of the parasitic capacitor 220 is Cgs, equivalent capacitance of the liquid crystal 217 is Clc, capacitance of the compensation capacitor 218 is Cstg, and the selection voltage of the scanning line is Vgon, xcex94Vgs, i.e., the voltage that has moved to the parasitic capacitor, is given by the following formula (1):
xcex94Vgs=(Cgs/(Cgs+Ccl+Cstg))xc3x97Vgonxe2x80x83xe2x80x83(1)
Consequently, it has been found that the voltage applied to the liquid crystal 217 drops by the amount of the voltage that has moved to the parasitic capacitor 220. This requires that Vcom 1, i.e., the opposed electrode voltage to be applied to the opposed electrode voltage feeding line, be shifted in advance down to a lower electric potential level by the amount of the voltage that will drop because of the influence of the parasitic capacitor 220.
Next, the explanation will be given below concerning operations of the respective voltage waveforms in pursuit of time. When applying VdWL, i.e., the white display voltage with negative polarity illustrated in FIG. 4A, if the selection voltage Vgon is applied to the scanning line, during a time-period xe2x80x9cT1xe2x80x9d, the source voltage vs is transitioned to the voltage level of the drain voltage Vd in the previous line (Vs is shifted up to a higher electric potential.). After that, if the opposed electrode voltage is alternated during a time-period xe2x80x9cT2xe2x80x9d, as illustrated in FIG. 4A, the source voltage Vs is shifted up to a higher electric potential in response to the alternating of the opposed electrode voltage. This is due to the fact that the variation in the opposed electrode voltage is steeper than writing speed of the TFT 216. After that, during time-periods xe2x80x9cT3xe2x80x9d, xe2x80x9cT4xe2x80x9d, the source voltage Vs is transitioned to the electric potential of the drain voltage Vd. Here, during the time-period xe2x80x9cT3xe2x80x9d, the source voltage Vs lies in a higher electric potential state than the opposed electrode voltage Vcom 2 inside the liquid crystal panel 206, and during the time-period xe2x80x9cT4xe2x80x9d, the source voltage Vs falls in a lower electric potential state than the opposed electrode voltage Vcom 2 inside the liquid crystal panel 206. In this operation, the source voltage electric potential is positioned at a considerably higher electric potential level as compared with the opposed electrode voltage electric potential. This increases the voltage distortion in the opposed electrode voltage, thus slowing down the convergence rate. Accordingly, in the timing with which the time-period xe2x80x9cT4xe2x80x9d is over, i.e., the timing with which the non-selection voltage Vgoff is applied to the scanning line, the electric potential difference between the source voltage Vs and the opposed electrode voltage Vcom 2 plays a role of VrmsWL 1, i.e., the effective voltage value to be applied to the liquid crystal 217. In the present conventional embodiment, the opposed electrode voltage Vcom 2 inside the liquid crystal panel 206 does not attain to the desired opposed electrode voltage Vcom 1. This generates an electric potential difference xcex94VcomH, which appears as a lack of the effective voltage value. Also, if the TFT 216 is transitioned to the xe2x80x9cOFFxe2x80x9d state, there occurs the above-described diving phenomenon of the voltage into the parasitic capacitor 220. This diving voltage level becomes equal to xcex94VgsWL. Thus, eventually, the resultant effective voltage value to be applied to the liquid crystal 217 becomes equal to xe2x88x92VrmsWL2 (=xe2x88x92VrmsWL1xe2x88x92xcex94VgsWH). Also, in this effective voltage value, there has occurred the lack of the effective voltage value that is equivalent to xcex94VcomH. This is because, as described above, the opposed electrode voltage Vcom 2 inside the liquid crystal panel 206 is lacked by the amount of xcex94VcomH as compared with the desired opposed electrode voltage Vcom 1.
Similarly, when applying VdWH, i.e., the white display voltage with positive polarity illustrated in FIG. 4B, if the selection voltage Vgon is applied to the scanning line, during a time-period xe2x80x9cT1xe2x80x9d, the source voltage vs is transitioned to the voltage level of the drain voltage Vd in the previous line (Vs is shifted up to a higher electric potential.). After that, during a time-period xe2x80x9cT2xe2x80x9d, alternating the opposed electrode voltage transitions the source voltage down to a lower electric potential voltage. Since the variation in the opposed electrode voltage is steeper than the writing speed of the TFT 216, as illustrated in FIG. 4B, the source voltage Vs is shifted down to the lower electric potential in response to the alternating of the opposed electrode voltage. After that, during time-periods xe2x80x9cT3xe2x80x9d, xe2x80x9cT4xe2x80x9d, the source voltage Vs is transitioned to the electric potential of the drain voltage Vd. Here, during the time-period xe2x80x9cT3xe2x80x9d, the source voltage Vs lies in a lower electric potential state than the opposed electrode voltage Vcom 2 inside the liquid crystal panel 206, and during the time-period xe2x80x9cT4xe2x80x9d, the source voltage Vs rises in a higher electric potential state than the opposed electrode voltage Vcom 2 inside the liquid crystal panel 206. In this operation, the electric potential difference between the opposed electrode voltage and the drain voltage has become larger than that in the above-described case illustrated in FIG. 4A where the tone voltage with negative polarity is applied. This condition increases amount of writing voltage during the time-period xe2x80x9cT4xe2x80x9d. Accordingly, the opposed electrode voltage Vcom 2 does not attain to the desired opposed electrode voltage Vcom 1, thus generating an electric potential difference xcex94VcomL. Consequently, the effective voltage value to be applied to the liquid crystal 217 becomes VrmsWH1, and thus it turns out that there has occurred a lack of the effective voltage value that is equivalent to xcex94VcomL. Also, if the TFT 216 is transitioned to the xe2x80x9cOFFxe2x80x9d state, there occurs the diving phenomenon of the voltage because of the above-described influence of the parasitic capacitor 220. This diving voltage level becomes equal to xcex94VgsWH. Thus, eventually, the resultant effective voltage value to be applied to the liquid crystal 217 becomes equal to VrmsWH2 (=VrmsWH1xe2x88x92xcex94VgsWL). Accordingly, it turns out that, on account of the insufficiency in the convergence of the opposed electrode voltage Vcom 2, there has occurred the lack of the effective voltage value that is equivalent to xcex94VcomL.
Next, referring to FIGS. 5A, 5B, the explanation will be given below regarding the manner in which the black display voltage is applied. When applying VdBL, i.e., the black display voltage with negative polarity illustrated in FIG. 5A, if the selection voltage Vgon is applied to the scanning line, during a time-period xe2x80x9cT1xe2x80x9d, the source voltage Vs is transitioned to the voltage level of the drain voltage Vd in the previous line (Vs is shifted up to a higher electric potential.). After that, during a time-period xe2x80x9cT2xe2x80x9d, the opposed electrode voltage is alternated and, as illustrated in FIG. 5A, the source voltage Vs is shifted up to a higher electric potential in response to the alternating of the opposed electrode voltage. After that, during a time-period xe2x80x9cT3xe2x80x9d, the source voltage Vs is transitioned to the electric potential of the drain voltage Vd. Then, during a time-period xe2x80x9cT4xe2x80x9d, the source voltage Vs is stabilized and, at this point in time, the effective voltage value applied to the liquid crystal 217 is VrmsBL1. Here, taking a diving voltage xcex94VgsBL into consideration, it is found that the effective voltage value applied to the liquid crystal 217 in the holding state becomes xe2x88x92VrmsBL2 (=VrmsBL1xe2x88x92xcex94VgSBL). Namely, this means the following: The opposed electrode voltage Vcom 1 has been shifted onto the lower electric potential side. Accordingly, when the TFT 216 lies in the xe2x80x9cONxe2x80x9d state, the source voltage Vs is positioned nearer to the positive polarity side than the opposed electrode voltages Vcom 1, Vcom 2. Then, the TFT 216 is transitioned to the xe2x80x9cOFFxe2x80x9d state. This causes the diving voltage to occur, thereby changing the effective voltage value VrmsBL1 into the tone voltage with the negative polarity. In addition to this, in comparison with the case illustrated in FIG. 4A where the white display voltage is applied, the electric potential variation in the source voltage Vs is infinitesimal during the time-period xe2x80x9cT2xe2x80x9d. This condition enhances the convergence rate at which the opposed electrode voltage Vcom 2 converges onto the desired opposed electrode voltage Vcom 1. Consequently, there occurs none of the variation in the effective voltage value on account of the insufficiency in the convergence of the opposed electrode voltage Vcom 2.
When applying VdBH, i.e., the black display voltage with positive polarity illustrated in FIG. 5B, if the selection voltage Vgon is applied to the scanning line, during a time-period xe2x80x9cT1xe2x80x9d, the source voltage Vs is transitioned to the voltage level of the drain voltage Vd in the previous line (Vs is shifted up to a higher electric potential.). After that, during a time-period xe2x80x9cT2xe2x80x9d, the opposed electrode voltage is alternated and, as illustrated in FIG. 5B, the source voltage Vs is shifted down to a lower electric potential in response to the alternating of the opposed electrode voltage. After that, during time-periods xe2x80x9cT3xe2x80x9d, xe2x80x9cT4xe2x80x9d, the source voltage Vs is transitioned to the electric potential of the drain voltage Vd, and the source voltage Vs is stabilized. In comparison with the case illustrated in FIG. 4B where the white display voltage is applied, the electric potential variations in the source voltage Vs and the opposed electrode voltage Vcom 2 are small enough. This condition decreases the amount of the writing voltage, thus enhancing the convergence rate at which the opposed electrode voltage Vcom 2 converges onto the desired opposed electrode voltage Vcom 1. Assuming that the effective voltage value applied to the liquid crystal 217 is VrmsBH1 during the time-period xe2x80x9cT4xe2x80x9d and taking a diving voltage xcex94VgsBH into consideration, it is found that the effective voltage value applied in the holding state becomes xe2x88x92VrmsBH2 (=VrmsBH1xe2x88x92xcex94VgsBH). As is the case with the description in FIG. 5A, there occurs none of the variation in the effective voltage value on account of the insufficiency in the convergence of the opposed electrode voltage Vcom 2.
In this way, in the state where the electric potential difference between the opposed electrode voltage Vcom and the drain voltage Vd is large, the voltage distortion in the opposed electrode voltage Vcom becomes larger. As the result, there occurs the lack of the effective voltage value toward the liquid crystal 217. Conversely, in the state where the electric potential difference between the opposed electrode voltage Vcom and the drain voltage Vd is small, the voltage distortion in the opposed electrode voltage Vcom becomes smaller. As the result, there occurs no lack of the effective voltage value toward the liquid crystal 217.
Next, the explanation will be given below regarding causes of the picture quality deterioration illustrated in FIG. 6, referring to FIGS. 7A, 7B and taking into consideration the voltage-applied state in the white display and the voltage-applied state in the black display.
In the lines in the regions (A) illustrated in FIG. 6, the white display data are included in the display data in the horizontal direction. This condition causes the opposed electrode voltage waveform Vcom 2 inside the liquid crystal panel 206 to take on a voltage waveform as illustrated in FIG. 7A. Namely, there occurs a voltage variation in the opposed electrode voltage Vcom 2 that is attributed to the white display, and thus there occurs a lack of the effective voltage value that is equivalent to xcex94VcomGH as compare with the desired opposed electrode voltage Vcom 1.
Similarly, in the lines in the regions (B) illustrated in FIG. 6, all the display data in the horizontal direction are the intermediate tone display data. This condition causes the opposed electrode voltage waveform Vcom 2 inside the liquid crystal panel 206 to take on a voltage waveform as illustrated in FIG. 7B. Namely, it turns out that the opposed electrode voltage Vcom 2 attains to the desired opposed electrode voltage Vcom 1. Accordingly, even in the same intermediate luminance display, the effective voltage value applied to the liquid crystal 217 in the holding state becomes different by the amount of xcex94VcomGH, and thus there occurs a lack of the effective voltage value. As the result, the white display data are included in the display data in the horizontal direction, and thus the intermediate luminance of the lines becomes dark display. Consequently, the difference in the effective voltage value, i.e., the amount equivalent to the insufficiency in the convergence of the opposed electrode voltage, is able to be recognized by the human eyes as the variation in the luminance, then being perceived as the transverse smear explained earlier.
Next, based on FIGS. 8 to 12, the further explanation will be given below concerning another conventional embodiment of a liquid crystal display. By the way, the configuration components in a circuit illustrated in FIG. 8 are the same as those in the conventional embodiment illustrated in FIG. 2 except for a compensation electrode 213. Accordingly, here, the explanation will be given regarding only the points that differ from those in the embodiment in FIG. 2.
In FIG. 9, a reference notation G1 denotes, of a group of scanning lines 214 illustrated in FIG. 8, a driving waveform of a scanning line that drives the 1st line. In addition, Vgon denotes a selection voltage level, and Vgoff denotes a non-selection voltage level. Similarly, G2 denotes a driving waveform of a scanning line that drives the 2nd line. Vcom indicates a driving waveform of an opposed electrode voltage signal line 212, and VcomP denotes a voltage level with positive polarity and VcomN denotes a voltage level with negative polarity. Vd indicates a tone voltage of a group of signal lines 213. When, with reference to the opposed electrode voltage Vcom, the tone voltage Vd is positioned on the negative polarity side, a voltage with negative polarity is applied to a pixel 215. Conversely, when the tone voltage Vd is positioned on the positive polarity side, a voltage with positive polarity is applied to the pixel 215.
The electric potential difference between the opposed electrode voltage Vcom and the tone voltage Vd causes the liquid crystal display to operate in such a manner that the luminance thereof is varied.
Additionally, in the conventional liquid crystal display illustrated in FIG. 8, there takes place a phenomenon that a voltage applied to a liquid crystal 217 causes a current leakage during a holding time-period, thus instabilizing a holding voltage of the liquid crystal 217. In order to prevent this phenomenon, in the liquid crystal display, there is commonly provided the compensation electrode 213. In addition, since a driving voltage for the compensation electrode 213 is also the same as a driving voltage waveform of the opposed electrode voltage Vcom, the description thereof will be omitted in the explanation in FIG. 9 and thereafter.
Next, referring to FIGS. 9 to 12, the explanation will be given below concerning problems in the conventional liquid crystal display. FIGS. 10A, 10B are diagrams for illustrating examples of displayed screens at the time of being displayed by the conventional liquid crystal display. FIG. 11 is a current path diagram for explaining causes of the picture quality deterioration in the conventional liquid crystal display. FIG. 12 is a driving waveform diagram for explaining the causes of the picture quality deterioration in the conventional liquid crystal display.
FIG. 10A is about an example of the following case: An intermediate gray is displayed all over the entire screen, and at the central portion, there are displayed a white rectangle, a light gray (gray the luminance (lightness) of which is higher as compared with that of the intermediate gray displayed all over the entire screen) rectangle, and a lighter gray (gray the luminance (lightness) of which is higher as compared with that of the above-mentioned light gray) rectangle. In FIG. 10A, there is illustrated the following phenomenon: Luminances of grays in display areas located on the right and the left sides of the three rectangles are lowered as compared with that of gray in the other display areas, and in addition, the respective amounts of the lowering in the luminances of the grays in the display areas located on the right and the left sides of the three types of rectangles are varied depending on the luminance levels of the three types of rectangles displayed at the central portion.
Similarly, FIG. 10B is about an example of the following case: Gray is displayed all over the entire screen, and at the central portion, there are displayed a black rectangle, a dark gray (gray the luminance (lightness) of which is lower as compared with that of the gray displayed all over the entire screen) rectangle, and a darker gray (gray the luminance (lightness) of which is lower as compared with that of the above-mentioned dark gray) rectangle. In FIG. 10B, there is illustrated the following phenomenon: Luminances of grays in display areas located on the right and the left sides of the three rectangles are heightened as compared with that of gray in the other display areas, and in addition, the respective amounts of the heightening in the luminances of the grays in the display areas located on the right and the left sides of the three types of rectangles are varied depending on the luminance levels of the three types of rectangles displayed at the central portion.
FIG. 11 illustrates electric current paths in the case where a voltage applied to each pixel on a line that a scanning line G1 selects has positive polarity, and indicates the manner in which the electric currents are concentrated onto the opposed electrode voltage signal line 212 and the compensation electrode 213.
In FIG. 12, a reference notation CL1 denotes a horizontal synchronization signal. The signal CL1 becomes effective at a ratio of one time during one horizontal time-period, and becomes a timing signal with which tone display data by the amount of one horizontal line are outputted by being transformed into a tone voltage. A notation M denotes the liquid crystal-applying voltage alternating signal. The signal M executes a control of converting the polarity of the opposed electrode voltage Vcom into negative polarity at the time of xe2x80x9cLowxe2x80x9d level and converting the polarity of the opposed electrode voltage Vcom into positive polarity at the time of xe2x80x9cHighxe2x80x9d level. A notation Vda denotes a tone voltage waveform that is applied to a portion of Da described in FIG. 10A and that is described by simplifying the portion of Da (i.e., described by reducing the number of the lines). A notation Vdb denotes a tone voltage waveform that is applied to a portion of Db described in FIG. 10A and that is described by simplifying the portion of Db (i.e., described by reducing the number of the lines).
Concerning the opposed electrode voltage Vcom, a full line (VcomA) indicates a waveform diagram of the opposed electrode voltage signal line 212 connected to an output terminal of a power supply circuit 205 illustrated in FIG. 8. Moreover, a dashed line (VcomB) indicates a waveform diagram inside a liquid crystal panel 206.
Next, referring to FIGS. 11, 12, the detailed explanation will be given below regarding the causes of the picture quality deterioration described in FIGS. 10A, 10B.
The luminances that the present conventional liquid crystal display displays are controlled by an effective voltage value Vdrms that is applied to the liquid crystal 217. The liquid crystal display is controlled so that, for example, when the effective voltage value is high, it displays a color of high luminance (white) and, when the effective voltage value is low, it displays a color of low luminance (black).
In FIG. 11, the opposed electrode voltage signal line 212 and the compensation electrode 213 are common to the respective pixels. Accordingly, in all the pixels, the electric currents are concentrated onto the opposed electrode voltage signal line 212 and the compensation electrode 213. When the electric currents are concentrated in this way, such loads as resistances (not illustrated) in the opposed electrode voltage signal line 212 and the compensation electrode 213 cause a voltage distortion to occur in the opposed electrode voltage and a compensation electrode voltage.
This voltage distortion has been found to be as illustrated in FIG. 12. Namely, during time-periods tH 1, tH 2 (both of which are the uppermost gray display areas of the rectangle region illustrated in FIG. 10A) and tH 9 (which is the lowermost gray display area of the rectangle region illustrated in FIG. 10A), the voltage level of the tone voltage remains constant (the tone voltage at the intermediate voltage level) in the horizontal direction as indicated by Vda, Vdb, and the opposed electrode voltage turns out to be expressed by VcomB. Meanwhile, during time-periods tH 3, tH 4 (both of which are the display areas of the white rectangle region illustrated in FIG. 10A), tH 5, tH 6 (both of which are the display areas of the lighter gray rectangle region illustrated in FIG. 10A) and tH 7, tH 8 (both of which are the display areas of the light gray rectangle region illustrated in FIG. 10A), Vda plays a role of executing the white rectangle display, the lighter gray rectangle display and the light gray rectangle display, which increases quantity of the electric current.
This condition increases quantity of the electric currents concentrated onto the opposed electrode voltage signal line 212 and the compensation electrode 213. This, further, prevents the opposed electrode voltage VcomB inside the liquid crystal panel 206 from attaining to the voltage level of the desired opposed electrode voltage VcomA. As the result, the opposed electrode voltage Vcom is lowered by the following amounts: xcex94Vcom 1 at the time of the white rectangle display, xcex94Vcom 2 at the time of the lighter gray rectangle display, and xcex94Vcom 3 at the time of the light gray rectangle display.
Also, in correspondence with the luminance levels of the three types of rectangles displayed at the central portion, the quantity of the electric currents concentrated onto the opposed electrode voltage signal line 212 and the compensation electrode 213 is also varied. Concretely speaking, the amount of the opposed electrode voltage Vcom that has been lowered as compared with the desired opposed electrode voltage VcomA is varied like +Vcom 1, xcex94Vcom 2, and xcex94Vcom 3. This results in a condition that effective voltage values, each of which has been lowered in the following way, are applied to the liquid crystal 217: With reference to the original effective voltage value Vdrms obtained during tH 1, tH 2 and tH 9, Vdrmsxe2x88x92xcex94Vcom 1 in the white rectangle display during tH 3, tH 4, Vdrmsxe2x88x92xcex94Vcom 2 in the lighter gray rectangle display during tH 5, tH 6, and Vdrmsxe2x88x92xcex94Vcom 3 in the light gray rectangle display during tH 7, tH 8.
The luminances that the liquid crystal display displays are controlled by the effective voltage value applied to the liquid crystal 217. Consequently, if the desired opposed electrode voltage fails to be obtained, the result is as follows: The display luminances are varied, and the luminances of grays in the display areas located on the right and the left sides of the three types of rectangle regions in the Db region described in FIG. 10A are lowered as compared with that of gray in the other display areas.
Meanwhile, as illustrated in FIG. 10B, there are provided the black rectangle region, the darker gray rectangle region and the dark gray rectangle region. This decreases quantity of the electric currents concentrated onto only the line region, thereby increasing the effective voltage value applied to the liquid crystal. Accordingly, there occurs the phenomenon that the luminances are heightened.
In this way, in the conventional liquid crystal display, the quantity of the electric currents concentrated onto the opposed electrode voltage signal line 212 and the compensation electrode 213 has been increased/decreased in correspondence with the display data. This has fluctuated the amount of the voltage distortion in the opposed electrode voltage and the compensation electrode voltage, thus causing the picture quality deterioration to occur.
The present invention has been made in view of the above-described problems. An object thereof is to provide a liquid crystal display that allows display of high picture quality to be embodied with the use of a low voltage driving circuit, and a driving method therefor.
It is an object of the present invention to provide a liquid crystal display that allows display of high picture quality to be embodied with the use of a low voltage driving circuit.
The present invention has been made in order to accomplish the above-described object. As the first mode thereof, there is provided a liquid crystal display including a liquid crystal panel having M units of pixels in the horizontal direction and N units of pixels in the vertical direction, each of the pixels including a switching element and a liquid crystal, a signal driving circuit for inputting display data and generating a tone voltage corresponding to the inputted display data so as to apply the tone voltage to one group of the pixels arranged in the horizontal direction and corresponding to the display data, and a scan driving circuit for selecting, in sequence, any one group of the pixels arranged in the vertical direction, the scan driving circuit, then, applying a selection voltage to the one group of the pixels arranged in the vertical direction and selected at that time and, meanwhile, applying a non-selection voltage to the other groups of the pixels arranged in the vertical direction and not selected at that time, the liquid crystal having, at one end thereof, an opposed electrode common to the respective pixels, wherein, when the selection voltage outputted by the scan driving circuit is applied to the switching element in each of the pixels, the tone voltage generated by the signal driving circuit is applied to the liquid crystal so that display luminance is controlled by an effective voltage value of the tone voltage toward the opposed electrode, and there are further included a circuit for detecting amount of data of the inputted display data and a power supply circuit for executing, in correspondence with the detected amount of the display data, an addition/subtraction control of a correction voltage value over an opposed electrode voltage value applied to the opposed electrode.
In the present invention, in order to accomplish the above-described object, there is provided a liquid crystal display apparatus or a driving method therefor, including a liquid crystal panel having M units of pixels in the horizontal direction and N units of pixels in the vertical direction, each of the pixels including a switching element and a liquid crystal, a signal driving circuit for inputting display data and generating a tone voltage corresponding to the inputted display data so as to apply the tone voltage to one group of the pixels arranged in the horizontal direction and corresponding to the display data, and a scan driving circuit for selecting, in sequence, any one group of the pixels arranged in the vertical direction, the scan driving circuit, then, applying a selection voltage to the one group of the pixels arranged in the vertical direction and selected at that time and, mean-while, applying a non-selection voltage to the other groups of the pixels arranged in the vertical direction and not selected at that time, the liquid crystal having, at one end thereof, an opposed electrode common to the respective pixels, wherein, when the selection voltage outputted by the scan driving circuit is applied to the switching element in each of the pixels, the tone voltage generated by the signal driving circuit is applied to the liquid crystal so that display luminance is controlled by an effective voltage value of the tone voltage toward the opposed electrode, and there are further included a member for detecting amount of data of the inputted display data and a voltage correcting member for executing a correction to the opposed electrode voltage value or an applying time-period thereof in correspondence with the detected amount of the display data and for each horizontal time-period.
Here, it is allowable that the voltage correcting member is configured to have, as an example, a circuit for generating a correction time-period controlling signal, the correction time-period controlling signal being generated for controlling the time-period during which the correction is executed to the opposed electrode voltage value in correspondence with the detected amount of the display data and for each horizontal time-period, and a circuit for executing an addition/subtraction control of a constant correction voltage value over the opposed electrode voltage value in correspondence with the correction time-period controlling signal generated and during only a time-period corresponding to the detected amount of the display data within each horizontal time-period. In this case, it is preferable that the circuit for generating the correction time-period controlling signal should include a data converting circuit including decoder circuits, a coincidence circuit, and a counter circuit. Also, it is preferable that the circuit for executing the addition/subtraction control of the correction voltage value should include an analog addition/subtraction circuit and an analog selecting circuit.
Also, it is allowable that the voltage correcting member is configured to have, as another example, a circuit for generating a correction time-period controlling signal for executing a correction to the opposed electrode voltage value during only a fixed time-period within one horizontal time-period, and a circuit for executing an addition/subtraction control of a correction voltage value over the opposed electrode voltage value in correspondence with the correction time-period controlling signal generated and during only a fixed time-period within one horizontal time-period. In this case, it is preferable that the circuit for generating the correction time-period controlling signal should include a counter circuit and a coincidence circuit. Also, it is preferable that the circuit for executing the addition/subtraction control of the correction voltage value should include a digital/analog converting circuit, an analog addition/subtraction circuit, and an analog selecting circuit.
Also, it is allowable that the voltage correcting member is configured to have, as still another example, a circuit for generating a correction time-period controlling signal, the correction time-period controlling signal being generated for controlling a time-period during which a correction is executed to the opposed electrode voltage value in correspondence with the detected amount of the display data and within one horizontal time-period, and a circuit for executing an addition/subtraction control of a correction voltage value over the opposed electrode voltage value during only a time-period corresponding to the correction time-period controlling signal generated, the correction voltage value corresponding to the detected amount of the display data. In this case, it is preferable that the circuit for generating the correction time-period controlling signal should include a data converting circuit including decoder circuits, a coincidence circuit, and a counter circuit. Also, it is preferable that the circuit for executing the addition/subtraction control of the correction voltage value should include a digital/analog converting circuit, an analog addition/subtraction circuit, and an analog selecting circuit.